A turbocharger is known as a supercharger used for improving the power output and the performance of an automobile engine. The turbocharger is an apparatus in which a turbine is driven by the exhaust energy of the engine to rotate a compressor with the power of the turbine, whereby the engine is supercharged to have more air fed into it than fed into it by natural suction. In the case of an engine capable of running at up to a high rotational speed region, when the engine is running at low rotational speeds, the exhaust turbine of the turbocharger hardly functions due to the reduced flow rate of the exhaust gas, so that the engine can not avoid giving a slow-moving feeling until the exhaust turbine runs efficiently, and necessitating a subsequent time or a so-called turbo-lag before the turbine rapidly reaches the full-running state. Furthermore, in the case of a diesel engine which runs inherently at low rotational speeds, there is a disadvantage that it is difficult to produce an effect of the turbocharger.
Therefore, a VGS turbocharger that works efficiently even when the engine is running at low rotational speeds has been developed. The turbocharger of this type is adapted to obtain a high power output when the engine is running at low rotational speeds by throttling flow of exhaust gas at a low flow rate with adjustable blades (vanes) to increase the velocity of the exhaust gas and increase work of an exhaust turbine. For this reason, in the VGS turbocharger, an adjusting mechanism for the adjustable blades are required additionally, and it is required that the associated constituting parts be formed to have a complicated shape or the like in comparison with those of the conventional one.
It has been common that, when an adjustable blade for such a VGS turbocharger is manufactured, a metal material (or a shaped material having a starting form for the adjustable blade) including a blade portion and a shaft portion that are integrally formed is first formed, for example, in accordance with a precision casting method represented by a lost wax casting method, a metal injection molding method or the like, and the shaped material is then suitably subjected to cutting or the like, to thereby finish the blade portion and the shaft portion to have desired shapes and dimensions.
However, in the technique using cutting operations applied to the shaped material, the following problems arise. Since this kind of turbo apparatus is constructed so as to introduce exhaust gas and utilize energy of the exhaust gas, a surface member thereof is naturally exposed to a high-temperature atmosphere of the exhaust gas. Since the exhaust gas contains constituents that can corrode the metal material, a heat resisting stainless steel such as SUS310S having excellent heat, oxidation resistance and the like is also used for the adjustable blade. However, since such a material is generally difficult to cut and therefore requires a lot of time to cut it, there is a problem that a lot of trouble is taken for processing it. In addition, since about ten to fifteen adjustable blades are required in one turbocharger, it is necessary to manufacture 300,000 to 450,000 adjustable blades a month in the case that about 30,000 motor vehicles are actually mass-produced monthly, so that the cutting operation is incapable of coping with this (production of about 500 adjustable blades is a limit in the cutting operation). In view of the foregoing, to mass-produce the adjustable blades, it is necessary to remove the cutting operation from the processing steps as much as possible, and rolling is mainly used for processing the shaft portion.
However, even in the case where the shaft portion of the adjustable blade is processed by rolling, there is a tendency that a shaft elongation (a phenomenon that the shaft portion is elongated in an axial direction), a sharp edge (a sharp portion is formed at a distal end of the shaft portion in a protruding state) and the like which are caused by rolling are increased, for example, when a rolling allowance is large. In some cases, in order to correct them, it is necessary to subject the shaft portion to cutting after rolling. Even in the cutting for correction, a lot of time is required as mentioned above, to thereby cause mass-productivity of the adjustable blade to be reduced. Accordingly, there has been required a manufacturing method which can restrict the shaft elongation caused by rolling as much as possible.
Additionally, recent years have seen tighter regulation of exhaust gas emission into the atmosphere particularly from diesel vehicles from a viewpoint of environmental protection and the like. For a diesel engine which inherently runs at low rotational speeds, mass production of a VGS turbocharger capable of improving the engine efficiency from a low rotational speed region has been strongly desired in order to reduce NOx, particulate matter (PM) and the like.